Many industrial processes, especially chemical processes, operate at elevated pressures. These processes often require a high pressure fluid feed, which may be a gas, a liquid or a slurry, to produce a fluid product or effluent. One way of providing a high pressure fluid feed to such an industrial process is by feeding a relatively low pressure stream through a pressure transfer device to exchange pressure between a high pressure stream and the low pressure feed stream. One particularly efficient type of pressure transfer device utilizes a rotor having axial channels wherein hydraulic communication between the high pressure fluid and the low pressure fluid is established in alternating sequences.
U.S. Pat. Nos. 4,887,942; 5,338,158; 6,537,035; 6,540,487; 6,659,731 and 6,773,226 illustrate rotary pressure transfer devices of the general type described herein for transferring pressure energy from one fluid to another. The operation of this type of device is a direct application of Pascal's Law: “Pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and to the walls of the containing vessel.” Pascal's Law holds that, if a high pressure fluid is brought into hydraulic contact with a low pressure fluid, the pressure of the high pressure fluid is reduced, the pressure of the low pressure fluid is increased, and such pressure exchange is accomplished with minimum mixing. A rotary pressure transfer device of the type of present interest applies Pascal's Law by alternately and sequentially (1) bringing a channel which contains a first lower pressure fluid into hydraulic contact with a second higher pressure fluid, thereby pressurizing the first fluid in the channel and causing an amount of first fluid that was in the channel to exit to the volumetric extent that higher pressure second fluid takes its place, and thereafter (2) bringing this channel into hydraulic contact with a second entrance chamber containing the incoming stream of lower first pressure fluid which slightly pressurizes the second fluid then in the channel, causing discharge of the second fluid at still lower pressure.
The net result of the pressure transfer process, in accordance with Pascal's Law, is to cause the pressures of the two fluids to approach each other. In a chemical process, such as sea water reverse osmosis which may, for example, operate at high pressures, e.g., 700-1200 pounds per square inch gauge (psig), a seawater feed may generally be available at a low pressure, e.g., atmospheric pressure to about 50 psig, and there will likely also be a high pressure brine stream from the process available at about 650-1150 psig. The low pressure seawater stream and the high pressure brine stream can both be fed to such a rotary pressure transfer device to advantageously pressurize the seawater while depressurizing the waste brine. The advantageous resultant effect of the rotary pressure transfer device in such an industrial process is a very substantial reduction in the amount of high pressure pumping capacity needed to raise the seawater feed stream to the high pressure desired for efficient operation; this can often result in an energy reduction of up to 65% for such a process and may allow a corresponding reduction in the required pump size.
In such a rotary pressure transfer device, there is generally a rotor with a plurality of parallel, open-ended channels. The rotor may be driven by an external force, but it is preferably driven by the directional entry of the fluid streams into the channels through an end cover, as known in this art. During rotation, there is hydraulic communication of the fluid residing in each channel, alternately and exclusively, with an inlet passageway either at one end of the rotor or at the other end. More specifically, the incoming higher pressure fluid enters the channel through an end cover from a high pressure inlet chamber, causing simultaneous discharge from the other end of the channel, and then, a very short interval later, the channel comes into exclusive communication with an incoming lower pressure fluid entering through the opposite end cover from an inlet chamber at the other end. As a result, axially countercurrent flow of fluids is alternately effected in each channel of the rotor, creating two discharge streams, for example a greatly increased pressure seawater stream and a greatly reduced pressure brine stream.
In a rotary pressure transfer device having such a rotating rotor, there will be many very brief intervals of hydraulic communication between the plurality of longitudinal channels in the rotor and both of the chambers at the opposite ends that are holding the two fluids, which chambers are otherwise hydraulically isolated from each other. Minimal mixing occurs within the channels because operation is such that the channels will each have a zone of relatively dead fluid that serves as a buffer or interface between the oppositely entering fluids; each fluid enters and exits from one respective end of the rotor. As a result, for example, a high pressure, higher salt concentration, brine stream can transfer its pressure to an incoming low pressure seawater stream without significant mixing.
In such prior art devices, the rotor usually rotates in a cylindrical sleeve, with its flat end faces slidingly and sealingly interfacing with the opposite end cover plates. The rotor in the pressure transfer device is often supported by a hydrostatic bearing and driven by either the streams of fluids entering the rotor channels or by a motor. To achieve extremely low friction operation, such a rotary pressure transfer device usually does not use rotating seals. Instead, fluid seals and fluid bearings are often used, and extremely close tolerance fits are used to minimize leakage.
The opposite end covers each have an inlet opening and a discharge opening that are located at diametrically opposed locations in their inward faces with which openings the channels in the rotor will alternately become aligned. The end covers are usually peripherally supported by contact with the sleeve within which the rotor revolves. Thus, the longitudinal rotor channels, at one end, alternately hydraulically connect with a high pressure brine inlet, for example, and then with a brine discharge outlet, while the same channel, at its other end, respectively hydraulically communicates with a high pressure seawater discharge outlet and then with a low pressure seawater inlet. In both instances, there is discharge of liquid from the opposite end of the channel through a discharge opening in the end cover to the extent that filling occurs at the feed end. As the rotor rotates between these intervals of alternate hydraulic communication, the channels are briefly sealed off from communication with either of the two openings in each end cover face.
In rotary pressure transfer devices of this general type, the volume of liquid that flows into and out of such a device as a result of rotation of the cylindrical rotor is at least partially dependent upon the total volume of the longitudinal channels in such a rotor and the ability to feed and withdraw liquid to and from those channels. Ways of increasing the efficiency of these pressure transfer devices have been continuously sought.